index.ts

import m from 'mithril'

m.render(document.body, m('h1', 'hello world'))

package.json

{
  "name": "mithriltypescript",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "build": "rollup -c"
  },
  "dependencies": {
    "mithril": "1.1.3",
    "rollup": "0.50.0",
    "rollup-plugin-commonjs": "8.2.1",
    "rollup-plugin-node-resolve": "3.0.0",
    "rollup-plugin-typescript": "0.8.1",
    "typescript": "2.5.2"
  }
}

rollup.config.js

import typescript from 'rollup-plugin-typescript'
import resolve from 'rollup-plugin-node-resolve'
import commonjs from 'rollup-plugin-commonjs'

export default {
  input: 'index.ts',
  output: {
    file: 'index.js',
    format: 'iife'
  },
  plugins: [
    resolve(),
    commonjs(),
    typescript()
  ]
}